Beckmann rearrangement template

Cyclohexanone oxime in acid: the ring C–C bond migrates to nitrogen as water leaves; water traps the nitrilium ion and tautomerisation gives caprolactam.

Mechanism step by step

An oxime rearranges to an amide; the group anti to OH migrates

1. Protonation – OH becomes a good leaving group
2. Migration – The anti C–C bond moves to N as water leaves
3. Water adds – Attack on the nitrilium carbon
4. Deprotonation – Imidic acid
5. N-Protonation – Tautomerisation begins
6. Amide – Loss of the O–H proton gives C=O

Key points

• Stereospecific: the group anti to the leaving group migrates.
• Caprolactam is the monomer of nylon-6 (millions of tonnes a year).
• Other promoters: PCl₅, SOCl₂, TsCl, polyphosphoric acid.
• Aldoximes tend to fragment to nitriles instead.

Drawn with 124 atoms, 21 arrows, 10 shapes – every one editable in MolDraw.

Sources

• Beckmann, Ber. Dtsch. Chem. Ges. 1886, 19, 988 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook